1. Field of the Invention
The present invention relates to a polarized light illuminator which unifies randomly polarized light (unpolarized light or natural light) emitted from a light source into unidirectionally polarized light having a uniform polarization direction, to illuminate an image display device such as a liquid crystal panel with the unidirectionally polarized light, and a projection type image display apparatus using such a polarized light illuminator.
2. Description of the Related Art
A polarized light illuminator as shown in FIG. 18 is disclosed in Japanese Laid-Open Publication No. 8-234205.
Referring to FIG. 18, a polarized light illuminator includes a light source 100, a rectangular prism 115, a wedge type prism 120, a first fly's eye lens 130, a second fly's eye lens 140, a light refracting prism 125, and a polarization conversion element 150 having .lambda./2 wave plates. A polarization separation film 180 composed of a dielectric multilayer film is formed on one surface of the wedge type prism 120 in contact with the rectangular prism 115 (i.e., between the rectangular prism 115 and the wedge type prism 120). A reflection film 190 made of evaporated Al is formed on the opposite surface of the wedge type prism 120. The polarization conversion element 150 is composed of .lambda./2 wave plates 200 and transparent regions 145. Light which has passed through the left half of each lens portion of the second fly's eye lens 140 passes through the corresponding transparent region 145, while light which has passed through the right half thereof passes through the corresponding .lambda./2 wave plate 200.
Functions of the above components will be described.
Randomly polarized light emitted from the light source 100 is incident on the rectangular prism 115. Only an S-polarized light component (a light component having a polarization direction perpendicular to the incident surface) of the incident light is reflected by the polarization separation film 180 formed between the rectangular prism 115 and the wedge type prism 120. A P-polarized light component (a light component having a polarization direction parallel to the incident surface) which has passed through the polarization separation film 180 is reflected by the reflection film 190 formed on the surface of the wedge type prism 120 opposite to the polarization separation film 180. The surface from which the S-polarized light is reflected (the surface of the wedge type prism 120 in contact with the rectangular prism 115) and the surface from which the P-polarized light is reflected (the opposite surface of the wedge type prism 120) have different angles with respect to the optical axis of the light source 100. Thus, the S-polarized light and the P-polarized light are incident on the first fly's eye lens 130 at different angles.
As a result, different converged points of the S-polarized light and the P-polarized light are formed side by side on the second fly's eye lens 140. The polarization conversion element 150 composed of the transparent regions 145 and the .lambda./2 wave plates 200 is disposed close to the second fly's eye lens 140 (on either the incident side or the output side thereof). The transparent regions 145 correspond to the portions of the second fly's eye lens 140 where the S-polarized light is output, while the .lambda./2 wave plates 200 corresponds to the portions thereof where the P-polarized light is output. Thus, the P-polarized light is converted into S-polarized light by the .lambda./2 wave plates 200. As a result, only S-polarized light is output from the polarization conversion element 150 and directed to a liquid crystal panel 170 via a plano-convex lens 160, to illuminate the liquid crystal panel 170 with the S-polarized light.
In this conventional polarized light illuminator, the randomly polarized light emitted from the light source 100 is unified into S-polarized light so that the liquid crystal panel 170 is illuminated with the unidirectionally polarized light. With such a construction, light emitted from the light source can be effectively utilized in a type of liquid crystal panel where images are displayed by modulating the polarization of light. However, the above construction requires a large glass prism, which increases the weight and cost of the device.
The above-cited Japanese Laid-Open Publication No. 8-234205 discloses another polarized light illuminator as shown in FIG. 19 to overcome the above problem.
Referring to FIG. 19, a polarized light illuminator includes a light source 100, a flat plate 110, a wedge type prism 120, a first fly's eye lens 130, a second fly's eye lens 140, and a polarization conversion element 150 having .lambda./2 wave plates. A polarization separation film 180 composed of a dielectric multilayer film is formed on one surface of the wedge type prism 120 in contact with the flat plate 110 (i.e., between the flat plate 110 and the wedge type prism 120). A reflection film 190 made of evaporated Al is formed on the opposite surface of the wedge type prism 120. The polarization conversion element 150 is composed of .lambda./2 wave plates 200 and transparent regions 145. Light which has passed through the left half of each lens portion of the second fly's eye lens 140 passes through the corresponding transparent region 145, while light which has passed through the right half thereof passes through the corresponding .lambda./2 wave plate 200.
Functions of the above components will be described.
Randomly polarized light emitted from the light source 100 is incident on the flat plate 110. Only an S-polarized light component of the incident light is reflected by the polarization separation film 180 formed between the flat plate 110 and the wedge type prism 120. A P-polarized light component which has passed through the polarization separation film 180 is reflected by the reflection film 190 formed on the opposite surface of the wedge type prism 120. The surface from which the S-polarized light is reflected (the surface of the wedge type prism 120 in contact with the flat plate 110) and the surface from which the P-polarized light is reflected (the opposite surface of the wedge type prism 120) have different angles with respect to the optical axis of the light source 100. Thus, the S-polarized light and the P-polarized light are incident on the first fly's eye lens 130 at different angles.
As a result, different converged points of the S-polarized light and the P-polarized light are formed side by side on the second fly's eye lens 140. The polarization conversion element 150 composed of the transparent regions 145 and the .lambda./2 wave plates 200 is disposed close to the second fly's eye lens 140 on the output side thereof. The transparent regions 145 correspond to the portions of the second fly's eye lens 140 where the S-polarized light is output, while the .lambda./2 wave plates 200 corresponds to the portions thereof where the P-polarized light is output. Thus, the P-polarized light is converted into S-polarized light by the .lambda./2 wave plates 200. As a result, only S-polarized light is output from the polarization conversion element 150 and directed to a liquid crystal panel 170 via a plano-convex lens 160, to illuminate the liquid crystal panel 170 with the S-polarized light.
Since no large prism is included, this conventional construction is advantageous over the construction shown in FIG. 18 in the aspects of weight and cost.
Another type of the polarized light illuminator as shown in FIG. 20 is described in "Monthly Display", Techno-Times Co., April 1997, pp. 100-104.
Referring to FIG. 20, a polarized light illuminator includes a light source 250, a first fly's eye lens 260, a rectangular prism 270, a flat prism 280, a second fly's eye lens 290, and a polarization conversion element 310 having .lambda./2 wave plates. A polarization separation film 340 composed of a dielectric multilayer film is formed on one surface of the flat prism 280 in contact with the rectangular prism 270 (i.e., between the rectangular prism 270 and the flat prism 280). A reflection film 350 is formed on the opposite surface of the flat prism 280. The polarization conversion element 310 is composed of .lambda./2 wave plates 300 and transparent regions 305.
Two adjacent lens portions of the second fly's eye lens 290 correspond to one lens portion of the first fly's eye lens 260. Light which has passed through one of any pair of adjacent lens portions of the second fly's eye lens 290 is incident on the transparent region 305 of the polarization conversion element 310, while light which has passed through the other of the pair of lens portions is incident on the .lambda./2 wave plates 300.
Functions of the above components will be described.
Light emitted from the light source 250 passes through the first fly's eye lens 260. Only an S-polarized light component of the incident light is reflected by the polarization separation film 340 formed between the rectangular prism 270 and the flat prism 280. A P-polarized light component which has passed through the polarization separation film 340 is reflected by the reflection film 350 formed on the opposite surface of the flat prism 280, and passes again through the polarization separation film 340. Thus, the optical axes of the S-polarized light and the P-polarized light are displaced with each other by an amount corresponding to the thickness of the flat prism 280, so that the S-polarized light and the P-polarized light are incident on adjacent lens portions of the second fly's eye lens 290. The polarization conversion element 310 composed of the transparent regions 305 and the .lambda./2 wave plates 300 is disposed close to the second fly's eye lens 290 (on either the incident side or the output side thereof). Each of the .lambda./2 wave plates 300 is aligned with the lens portion of the second fly's eye lens 290 through which the P-polarized light passes, while each of the transparent regions 305 is aligned with the lens portion through which the S-polarized light passes. Only the P-polarized light is therefore converted into S-polarized light by the .lambda./2 wave plate 300. As a result, the liquid crystal panel 330 is illuminated with only S-polarized light via the plano-convex lens 320.
In the conventional polarized light illuminators described above, light emitted from a light source can be converted into unidirectionally polarized light. Using this technique, light from a light source can be effectively utilized when an image display device is used, such as a liquid crystal panel of a birefringence mode or a twisted mode, where images are displayed by modulating the polarization of light.
The above-described conventional polarized light illuminators have the following problems.
The conventional polarized light illuminator shown in FIG. 19 is disadvantageous in the following points. FIG. 21 is a schematic view for explaining the optical paths of the P-polarized light and the S-polarized light for the polarized light illuminator of FIG. 19. This specific polarized light illuminator shown in FIG. 21 is constructed so that the P-polarized light and the S-polarized light are incident on the first fly's eye lens 130 at angles displaced by .+-.3.degree. with respect to the normal of the surface of the first fly's eye lens 130. Assuming that the refractive indexes of the flat plate 110 and the wedge type prism 120 are 1.52 (BK7), the polarization separation film 180 formed on the flat plate 110 and the surface of the wedge type prism 120 on which the reflection film 190 is formed are tilted by 46.5.degree. and 45.degree., respectively, with respect to the optical axis of the light source 100.
With the above construction, the optical paths of the P-polarized light and the S-polarized light should desirably intersect with each other on the incident surface of the first fly's eye lens 130. In reality, however, as shown in FIG. 19, the optical paths of the P-polarized light and the S-polarized light output from the wedge type prism 120 are displaced more the further they are from the prism. As a result, when the first fly's eye lens 130 has a width identical to the output beam diameter of the light source 100, part of the light fails to be incident on the first fly's eye lens 130 (a hatched portion 20 in FIG. 19), resulting in light loss. On the other hand, when the width of the first fly's eye lens 130 is made larger so that the portion of light corresponding to the hatched portion 20 can be incident on the first fly's eye lens 130, the following problem arises. That is, the portion of light output from the end portion of the widened first fly's eye lens 130 is directed to enter the liquid crystal panel 170 at a large incident angle. Such light will not be received by a projection lens.
Such a failure to realize the intersection between the optical paths of the light reflected by the polarization separation film and the light reflected by the reflection film on the first fly's eye lens 130 is a problem specific to the device using the combination of the flat plate 110 and the wedge type prism 120. This also applies to the construction as shown in FIG. 22 where the top of the wedge is located in reverse.
The top portion of the wedge type prism 120 is beveled to protect against cracking, breaking, and the like at the end faces. Such beveling increases the distance between the polarization separation surface and the reflection surface. As a result, the intersection between the optical paths of the light reflected by the polarization separation surface and the light reflected by the reflection surface fails to be located right on the first fly's eye lens, which generates light loss.
The wedge type prism 120 is generally produced by grinding a flat prism. The cost is therefore high compared with the case where a flat prism is used. Thus, using the wedge type prism causes a number of problems as described above.
The conventional polarized light illuminator shown in FIG. 20 is disadvantageous in the following points. A polarized light illuminator should desirably provide light from a light source to a liquid crystal panel without light loss. In reality, however, light is lost due to a variety of factors. One of such factors is the reflection of light from a glass surface. About 4% of light is reflected from a glass surface unless the glass surface is subjected to anti-reflection (AR) coating. It is therefore desirable to minimize the use of glass surfaces in an optical path. Although the reflection of light from the glass surface can be reduced by surface treatment such as AR coating, this increases the cost.
In the construction shown in FIG. 20, light is lost at an incident surface 360 and an output surface 370 of the rectangular prism 270. If such surfaces are subjected to AR coating to avoid the light loss, the cost increases. Therefore, the number of optical faces should be as small as possible.
The rectangular prism 270 is required to have a size corresponding to the diameter of a reflector 251 for the light source 250. A normal polarized light illuminator uses a reflector with a diameter of about 80 to 100 mm for a light source. In order to meet this size, a large rectangular prism is required. This increases the weight and the cost of the device.
In the three conventional polarized light illuminators described above, the polarization separation film composed of a normal dielectric multilayer film is used. In general, a greater effect is obtained when S-polarized light is reflected from the polarization separation film. These conventional polarized light illuminators are therefore constructed to have this effect. However, in the case where the alignment direction of liquid crystal in the image display device is tilted with respect to the polarization direction of the S-polarized light or the P-polarized light, both the light reflected by the polarization separation film and the light reflected by the reflection film are required to pass through the wave plate so as to convert the polarization directions thereof. The wave plate, however, does not convert the polarization direction of all the light over the visible wavelength range because it has a wavelength dependence. A light component which has not been converted into light with a desirable polarization direction is not usable for the image display device, resulting in light loss. Light is also lost due to the absorption by the wave plate when the light passes through the wave plate.